1. Field of the Invention
The present invention relates to a disk drive system having means for optimizing power consumption efficiency during a seek operation. More particularly, the present invention relates to a method for reducing spindle driving current while actuator driving current reaches a peak during the deceleration sequence of an actuator seek operation.
2. Description of the Prior Art
Modern disk drive systems typically include: a disk having a thin magnetic coating upon which user data and position information is stored in a series of concentric or spiral tracks; a spindle motor assembly for supporting and rotating the disk in response to a spindle current provided by a spindle driver circuit; a read/write head for generating and detecting variations in magnetic orientation of the magnetic material as the disk is rotated relative to the head; a head-arm assembly for supporting and moving the head radially over the surface of the disk; and an actuator, usually comprising a voice coil motor, for driving the head-arm assembly in response to an actuator current provided by an actuator driver circuit. Disk drive systems also include a channel unit, having a read/write channel and a position error channel, electrically coupled to the read/write head via an arm electronics unit; and a servo controller unit responsive to a position error signal received from the head via the channel unit, and operative to provide an actuator control signal to the actuator driver circuit for controlling the actuator current, and further operative to provide a spindle command signal to the spindle driver circuit for controlling the spindle current.
The position error signal, which is proportional to the relative difference of the positions of the center of the servo head and the nearest track center, provides information relating to sources of motion including motion of the actuator and motion of the disk surface itself. The servo system, whether of a dedicated servo type or an embedded servo type, uses the position error signal to keep track of actuator position as well as velocity.
The servo system may be analog or digital. Digital servo control systems are discrete time systems in which position information is sampled. Modern digital servo controllers, which may be implemented by a microprocessor or a digital signal processor, use state estimators to determine velocity. Analog servo systems use either a velocity transducer or some type of estimating scheme to determine velocity parameters. A typical analog velocity estimator uses both a filtered integral of acceleration and a filtered derivative of position to obtain velocity feed back.
Servo systems may operate in several modes generally including a track following mode and a seek mode. In the track following mode, the servo controller maintains the head in a path over the centerline of a selected track to facilitate accurate reading and recording of data in the track. In the seek mode, the servo controller is directed to place the head on a target track different from the present track. A seek operation includes an acceleration sequence and a deceleration sequence. During each of these sequences, the current provided by the actuator driver circuit to the actuator is increased to a peak value which is greater than the average current value. The seeking performance of a servo system is proportional to the amount of power available for providing current to the actuator during acceleration and deceleration.
Typically, a single power supply provides power to both the spindle driver circuit and the actuator driver circuit. High performance files require a high spindle velocity which necessitates a high spindle current level. High performance files also feature fast seeking performance which requires high peak actuator current levels during acceleration and deceleration. The peak actuator current drawn during the deceleration sequence of the seek is significantly more than the peak current drawn during the acceleration sequence due to voltage adding effects of a back electromotive force (back EMF) induced in the actuator. During the deceleration sequence, back EMF effectively adds to the supply voltage so that more actuator current can be drawn from the power supply. Conversely, during the accelerate portion of the seek, the back EMF subtracts from the supply voltage, thereby causing the current to become lower as the actuator moves faster. In modem servo controller designs, velocity versus position schedulers of the servo controller are designed assuming that the acceleration and deceleration will change with the velocity of the actuator.
Although the back EMF provides the ability to decelerate faster and thus improve seek times, the peak actuator current drawn during deceleration is increased. An excessive peak actuator current drawn during deceleration presents problems because the peak power output of the disk drive power supply, which concurrently provides power to the spindle driver, may be exceeded when the peak deceleration current is reached. For a disk drive including multiple platters and having data stored according to a striping technique, the peak current drawn during deceleration is particularly problematic because each of a plurality of actuators may seek, and decelerate, at the same time thereby drawing peak currents simultaneously.
Because seek performance is limited by the amount of current available for acceleration and deceleration, there is an inherent design tradeoff between seeking performance and peak actuator current. One way to ensure that the peak power output of the power supply is not exceeded during deceleration is to design the deceleration profile with adequate margin for peak current taking into account the maximum output of the power supply, back EMF effects of the actuator, and other appropriate design parameters. Increasing the coil resistance of the actuator provides reduction of the peak actuator current values, but this is accomplished at the cost of decreased seeking performance. Therefore, as the seek performance and spindle speeds required by higher performance files increase, it becomes necessary for disk drives to be equipped with larger power supplies which is very costly.
Dunn (U.S. Pat. No. 5,381,279) discloses a method of reducing peak power requirements of a power supply of a disk drive wherein the spindle current is reduced during the acceleration sequence of the seek operation, and then compensated for during a coast sequence of the seek operation so that the spindle velocity is maintained at the required rate. However, this method does not provide a solution to the above described problem of high peak actuator current drawn during the deceleration sequence due to back EMF effects. Also, not all seek operations include a coasting sequence during which to compensate for spindle current is reduced during actuator acceleration.
What is needed is a method and apparatus for limiting the total amount of power drawn from the power supply of a disk drive during the deceleration sequence of a seeking operation wherein seeking performance is not sacrificed.